1. Field of the Invention
The present invention relates to composite powders based on silver-tin oxide, and to methods for making them. The invention relates also to electrical contact materials made from such composite powders.
2. Description of the Related Art
Electrical contact materials typically consist of silver with certain metal and/or oxide additives. These materials are made using powder metallurgy techniques.
The materials are chosen based on the intended use, such as the type of switching device, the switching current, and the electrical load. General requirements include low electrical wear with high arc resistance, and low weld force with low contact resistance.
The air contactors of low voltage/high current equipment, within a switching current range of 100-3000 A, are primarily made with materials based on silver-tin oxide, in which the oxide content, in practice, lies between about 8 and 12 wt. %. Contact materials of this type generally have acceptable arc resistance; sufficient safety with respect to contact welding; comparable low material migration with low contact resistance and good overtemperature behavior; and practical processing and jointing properties.
Further improvements in the processing and contact properties of these Agxe2x80x94SnO2 materials are desired, as is an increase in their range of applications. Further developments in materials and technology are therefore to be expected. Such developments might well involve variations in the primary oxides, the other oxide additives, and the manufacturing technology, particularly technology aimed at controlling the structure-dependent properties.
The oxide components used are typically selected with the goal of improving the contact properties, thus reducing the specific contact erosion. Other goals include improving the contact resistance, the overtemperature and the weld force and weld frequency. These are prerequisites for obtaining a high load carrying capacity, a greater lifetime, and improved reliability of the contact system. Typical oxide additives used for contact materials based on Agxe2x80x94SnO2 include tungsten oxide, molybdenum oxide, bismuth oxide, copper oxide and indium oxide, used individually or in combination on the basis of their specific effects. These oxides are chosen mainly based on their thermodynamic properties, as well as on their wetting behavior in the Agliquid/SnO2 system (Jeannot, et al., IEEE Proceedings Holm Conference 1993, p. 51).
Silver oxide powders used as precursors for contact materials are typically made using one of the following processes: powder metallurgy mixing techniques; internal oxidation of alloying powders or compact bodies under elevated oxygen partial pressure; and the chemically reductive precipitation of some or all of the components of the material. Further processing of the composite powders to semi-finished contacts or contact units, as a rule, takes place by cold isostatic compaction of the powders, followed by sintering and extrusion, and reforming to the end size.
The powder metallurgy mixing techniques for producing composite powders consist of mechanical homogenization of solid starting substances in powder form in a mixer, for the most part using only the silver and the oxide additive, but frequently also adding other additives or sintering aids. The method can be used either wet or dry, for instance with water, alcohol, etc., but is limited to powders having a particle size greater than 1 xcexcm. The conventional mixing technique runs up against the technical limits in manufacturing composite powders having extremely finely divided oxide distributions, because of the particle and granule sizes, as well as because of the more or less pronounced agglomerate formation.
Because of the need for a homogeneous, very finely dispersed microstructure of the oxide phases, either the internal oxidation method or the chemically reductive precipitation method is used. The preferred contact materials are silver-tin oxide doped with indium oxide, having a homogeneity as high as possible. Manufacture of these preferred contact materials has up to now taken place only using the internal oxidation process, which is costly.
In the internal oxidation process, the formation of the oxide additive takes place either on an alloying powder atomized from the melt, or on the product produced by powder metallurgy or melt metallurgy. However, this technique can only be used with the typically used oxides if special measures are taken. If the external oxidation phenomena are successfully suppressed, which otherwise lead to passivation of the process, oxide particles with particle sizes around 100 nm can be produced.
In the process of chemically reductive precipitation, in contrast, the components of the material are precipitated from an ionic solution. This can involve either the complete precipitation of all of the material components, including the oxides; or precipitation of silver onto components suspended in an aqueous solution. In the first variation, the distribution of the components is dependent on the reaction kinetics. In the second variation, the particle size of the suspended components is the determining factor for the microstructural fineness of the end product.
These different processes produce different structural formations in the metal and oxide phases of the silver matrix; produce significant changes in the structure-dependent material properties; and thus produce significant changes in the processing and contact properties, which are difficult to predict in terms of size and tendency.
Chemically reductive processes of composite powder manufacture which have been recently developed are based chiefly on the principle of precipitation of silver onto oxides suspended in an aqueous solution. However, these processes differ with regard to the oxides used and their particle sizes, the precipitation systems, and the course of the reaction. Thus, considerable differences with respect to the quality of the composite powders necessarily result from different system- and process-related structural formations of the oxide phases of the silver matrix.
EP 0 370 891 describes the manufacture of contact materials from silver-tin oxide particles which optionally contain additional small amounts of copper oxide as dopant. These particles are obtained by adding a strong base to a silver nitrate solution, containing tin oxide having a specific particle size, in order to precipitate the silver oxide onto the tin oxide particles. In a further step, the resulting powder is heated in order to reduce the silver oxide to metallic silver. However, this method is limited with respect to the choice of doping agents, since many of the doping materials of interest for contact materials dissolve in a highly basic environment and thus do not reappear in the precipitation product.
U.S. Pat. No. 5,846,288 describes the manufacture of composite powders by precipitation of silver onto certain oxide base materials that are optionally doped with selected elements. Compaction, breaking, and grinding operations which follow the precipitation are necessary in order to obtain from the precipitation product a homogeneous and free-flowing powder from which compact contact materials can then be made. The precipitation process takes place either so that a suspension of the oxide in a silver nitrate solution is sprayed into a reactor containing a reduction agent (hydrazine hydrate is disclosed) or, vice versa, hydrazine hydrate is sprayed into a reaction vessel containing a suspension of the oxide in a silver nitrate solution.
Hydrazine is known to be dangerous to human health and the environment. The disclosed method has the additional disadvantage that it produces a considerable fraction of finely divided silver particles that are isolated and thus are not bonded to oxide particles. This has a fundamental adverse affect on the homogeneity of the composite powder. Also, in the further processing to a compact material, it turns out that coarser silver clusters form to a considerable extent.
The present invention has the goal of influencing the processing and contact properties of contact materials based on Agxe2x80x94SnO2, having an essentially conventional composition, by the appropriate design of the manufacturing process, and in particular, with the goal of maximum homogeneity and particle fineness. Above all, the intention was to be able to use a wide choice of dopants, and in particular, to use indium oxide.
The present invention obtains surprising improvements in the processing and contact properties of contact materials based on silver-tin oxide made using the powder metallurgical process. Specifically, the invention relates to a silver-tin oxide composite powder obtained by chemically reductive precipitation of silver onto particulate tin oxide. The precipitation is carried out so that a solution of a silver compound and a solution of a reducing agent are added to an aqueous suspension of tin oxide simultaneously, in stoichiometrically equivalent amounts, separately and continuously over the course of the reaction, with intensive mixing.
The invention thus relates also to methods for producing such composite powders based on silver-tin oxide, by chemically reductive precipitation of silver onto particulate tin oxide. The method is characterized in that a solution of a silver compound and a solution of a reducing agent are added to an aqueous suspension of tin oxide simultaneously, in stoichiometrically equivalent amounts, separately and continuously over the course of the reaction, to reductively precipitate the silver onto the particulate tin oxide. Such reducing agents are therefore also known as precipitation agents.
The invention also relates to the use of composite powders made in this way for the powder metallurgical manufacture of contact materials based on silver-tin oxide.